This invention relates to a mechanical driver. Said mechanical driver can be miniature mechanical driver and is often used in a micro-pump. The micro-pump can be used as the fluid pumping device of a drug delivery system.
A variety of mechanical drivers have been described for providing the mechanical displacement required in devices designed for pumping fluids. Examples of these mechanical drivers include devices operating on thermo-pneumatic (U.S. Pat. No. 4,265,600 and U.S. Pat. No. 6,520,753), electrostatic (U.S. Pat. No. 6,168,395 and U.S. Pat. No. 5,362,213), piezo electric (U.S. Pat. No. 4,596,575 and U.S. Pat. No. 6,827,559), thermo-hydraulic (GB2443261), bimetallic (U.S. Pat. No. 5,611,676), stepper motors (EP2072072) and magnetic (U.S. Pat. No. 3,819,305 and U.S. Pat. No. 7,922,462) mechanical driving principles.
A number of limitations exist with these mechanical driving principles when incorporated into micro-pump designs. A number of these mechanical drivers a too complex and lead to difficulties when designing a product where large numbers need to be manufactured, that are manufactured at high throughput and where the manufacturing process is required to deliver product at a cost effective price. The complexity also limits the design opportunities when they are incorporated into micro-pumps. Other limitations are that the components required in some of the mechanical drivers described above result in a product that is too costly or difficult to manufacture. Yet another limitation is that the materials required when incorporating at least some of the mechanical drivers mentioned above into micro-pump products reduce the opportunities for using the product. As an example, some materials are not compatible with the fluid media that the product is required to pump, because it degrades the commercially important components in the media. This could include commercially important components such as bioactive materials. Yet in other instances, some of the mechanical driving principles are not able to provide the accuracy required for micro-pumps required for drug delivery and other commercially important components. And yet another limitation is that some of the above mechanical driving principles are not able to provide the repeatability required by certain drug delivery products. By way of example, products designed for short term use and that are replaced on a frequent basis require driving principles that can provide repeatable performance across a large number of similar devices. And yet another limitation is that some of the above mechanical driving principles do not provide the power required by the drug delivery device when the mechanical driver is miniaturised.
Shape memory alloy (SMA) has been proposed as a suitable material for a mechanical driver of the type described above and a number of devices based on this mechanical driver have been described. SMA mechanical drivers can be suitable for micro-pump applications due to their high force-to-weight ratio, mechanical simplicity, compactness, and silent, clean operation. SMA mechanical drivers also provide cost effective solutions for the design of short term use, disposable products that are easy to manufacture, that are produced in very large numbers and at a cost effective price.
However, SMA mechanical drivers have disadvantages that limit their use in applications that require high accuracy.
One disadvantage of using SMA in these devices is that it has a prominent strain hysteresis and its phase transition is dependent on temperature, stress, the direction of motion, and many other factors (J. D. Harrison, “Measurable Change Concomitant with SME Transformation,” Engineering Aspects of SMAs, eds. Duering et al., Butterworth, pp 106-209, 1990).
Nonlinear control approaches have been used to compensate for the non-mechanical non-linearity of shape memory alloys. These approaches have included various approaches to controlling the mechanical movement of shape memory alloys such as: neural networks and a sliding mode based robust controller (Song, “Precision tracking control of shape memory alloy actuators using neural networks and a sliding-mode based robust controller,” Smart Mater. Struct. 12, pp. 223-231, 2003), neural fuzzy (Kumagai, “Neuro-fuzzy model based feedback controller for shape memory alloy actuators,” Proceedings of SPIE, v 3984, pp. 291-9, 2000) dissipativity (Gorbet, “Dissipativity approach to stability of a shape memory alloy position control system,” IEEE Transactions on Control Systems Technology, v 6, n 4, pp. 554-562, July 1998), variable structure control (Grant, “Variable structure control of shape memory alloy actuators,” IEEE Control Systems Magazine, v 17, n 3, pp. 80-88, June 1997), and pulse width modulation of the actuation energy (NMa and G Song, “Control of shape memory alloy actuator using pulse width modulation,” Smart Mater. Struct. 12, pp. 712-719, 2003). Despite these often complex approaches to shape memory alloy control, the control of SMA is still difficult.
Several approaches have also been proposed to generate the accuracy of movement required from SMA mechanical drivers by mechanically limiting the range of movement that the SMA can perform. EP2290238A1 describes a device that limits the range of movement of a plunger in a fluid delivery device by proving mechanical stops for both the start and end of the plunger travel. U.S. Pat. No. 7,232,423 describes a device that also uses mechanical stops to accurately define the range of movement created by the SMA mechanical driver. A limitation of these inventions is that these mechanical stops impart strain on the SMA and limit the performance of the driver and could also lead to failure. U.S. Pat. No. 8,047,812 describes a device that aims to reduce the effect of unwanted strain on the SMA by introducing a second piston coupled to the shape memory element that moves to accommodate changes in the shape memory element and reduce stress on the pumping system. By introducing the second piston, this invention increases the complexity of the device, making it more difficult to manufacture and less cost effective. U.S. Pat. No. 8,029,245 describes a device that relies on monitoring the position of the piston in the pumping system and then modulating the energy supplied to the SMA to provide the accuracy required. The requirement for monitoring the position of the plunger in this invention introduces the need for complex sensor and control systems that complicate the design and operation of the device. These added complications also increase the cost and complexity of manufacturing the device. U.S. Pat. No. 6,656,158 describes a fluid dispensing device that uses a SMA to move a pawl against a toothed gear system attached to the fluid dispensing portion of the device. Every time the SMA is activated the pawl moves against the gear and indexes the gear from its first position to a second position. The gear does not return to its first position. This device overcomes the lack of accuracy in the use of SMA, by using the SMA to move an accurately formed gear system. U.S. Pat. No. 6,375,638 describes a device that is similar to the one described in U.S. Pat. No. 6,656,158. U.S. Pat. No. 6,375,638 describes a device where the SMA is used to move a part that then deflects a second part from its first position to a second position. The part that is moved can either move in a linear motion or an angular motion. It is important to note, that this part does not return to its first position, but indexes along the path of travel every time the SMA is activated. In both U.S. Pat. No. 6,656,158 and U.S. Pat. No. 6,375,638, the complexity of the device described increases the complexity of the manufacturing process and the cost-effectiveness of the manufactured device.
There is a need for an improved shape memory actuator mechanical driver that provides the required accuracy, reliability, ease of manufacture, cost effectiveness and that is scalable and that can be used to drive the reciprocating piston in a micro pump. These will become apparent in the description of the present invention.